The present invention relates in general to particle detection by light scattering techniques and, more particularly, to the characterization of particles by observing the phase and amplitude of the beat frequency resulting from the interference of the light scattered as a result of the interaction of two collinear, orthogonally polarized light beams having a small difference in wavelength with the particles under investigation as a function of scattering angle. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Phase and polarization properties of elastically scattered light have received increased attention in recent years since they provide more information about the size, morphology, optical activity, and internal structure of the scattering particle than do nonpolarization measurements of the scattered irradiance. The Mueller scattering matrix is commonly used to characterize elastic light scattering from particles. The 16 real elements of this matrix specify the intensity and polarization of the scattered light. Although only the irradiances of various polarizations need be measured in experiments designed to determine individual Mueller matrix elements it is difficult to separate, as a practical matter, contributions from other Mueller matrix elements to the element under investigation. Wavelength and angular parameterization of the Mueller matrix elements can, in select situations, provide a significant amount of information about the scattering particle or particles.
The amplitude scattering matrix, by contrast, has received little attention since it explicitly relates the phases of the incident and scattered electric fields. Experimental phase measurements have traditionally proven to be difficult since interferometric techniques are required.
Two-frequency Zeeman effect lasers and frequency-stabilized lasers have recently become available commercially. As a result, certain types of phase information in the scattered light are now readily accessible. When the orthogonally polarized, binary output radiation from such a device is scattered by a sample of particles or a single particle of interest in a flow, and the resulting two scattered frequencies are made to interfere on a photodetector, after passing through a polarization element, the phase and amplitude of the resulting beat frequency contains useful information concerning the nature of the scattering particles. The term, phase differential scattering, PDS, will be used to refer to the phase and amplitude measurements made when a sample of particles in a flow or individual particles scatter orthogonally polarized, two-frequency electromagnetic radiation. Phase differential scattering is sensitve to three scattering mechanisms, each of which depends upon the nature of the scatterer. All three mechanisms may occur simultaneously from a single scattering particle. At a given scattering angle, the scattering particle may scatter the two incident radiations with different efficiencies. This can be measured by setting the transmission axis of the analyzing polarizer at 45.degree. to the horizontal and recording the amplitude of the beat frequency between the two wavelengths of the radiations. The scattering particle may also retard one of the two scattered radiations relative to the other. This phenomenon manifests itself in the phase of the beat frequency when the analyzing polarizer is oriented at 45.degree.. The scatterer may also partially convert one orthogonal polarization of the radiations into the other. The extent and relative retardation of this "mixing" of polarizations depends upon the asymmetry of the particles. It can be characterized by measuring the amplitude and phase, respectively, of the beat frequency when the analyzing polarizer is set to 0.degree. and 90.degree..
Accordingly, it is an object of the present invention to provide an apparatus and method for accessing much of the information contained in the Mueller scattering matrix or theoretically available from conventional interferometric measurements on light scattered from a sample of particles or from individual particles under investigation.
Another object of our invention is to provide an apparatus and method for identifying and characterizing particles in samples.
Yet another object of the subject invention is to provide an apparatus and method for identifying and characterizing individual particles in a flowing system.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.